With the recent development of miniaturized high frequency communication devices such as portable telephones, there is a demand for piezoelectric acoustic wave devices (resonators and filters), used for these communication devices, which are smaller and have a higher frequency.
Conventional piezoelectric acoustic wave devices have been miniaturized by improving ways to support the packages and piezoelectric plates. The piezoelectric plates of conventional piezoelectric acoustic wave devices have been thinned to raise the frequencies of the devices. One conventional piezoelectric acoustic wave device, a quartz resonator, is explained by referring to FIG. 6.
FIG. 6 is a cross-sectional view of a conventional quartz resonator. In FIG. 6, 51 is a quartz plate abraded to a thickness at which the plate can resonate at a preferable frequency. Exciting electrodes 52 are fixed to both surfaces of the quartz plate. Conductive adhesive 53 is used to fix two sides of quartz plate 51 to a package 54 while maintaining continuity between exciting electrodes 52 and a lead electrode 55.
A piezoelectric acoustic wave device cannot operate or has poor properties unless the resonating part is highly free from the substrate and the like. Therefore, in this conventional quartz resonator, quartz plate 51 is supported by fixing only two sides of the plate to package 54.
Since it is extremely difficult to treat an extremely thin piezoelectric plate with photolithography, film forming processes, mounting processes and the like, an acceptable minimum thickness of the plate is about 50 .mu.m. The thickness of the piezoelectric plate should be at least about 50 .mu.m so that the plate cannot be bent or cracked during an abrading treatment.
Quartz plates generally used for high frequency resonators are AT-cut quartz plates since the plates have small frequency deviations, which are caused by a change in temperature. The resonant frequency is in inverse proportion to the thickness of the quartz plate, and the fundamental resonant frequency of an about 50 .mu.m thick quartz plate is 35 MHz. In other words, it is difficult to manufacture a quartz plate with a resonant frequency higher than 35 MHz. Thinning a quartz plate also becomes more difficult as the area of the plate becomes large.
Since a conductive adhesive spreads during a coating step, the size of a device is required to be large enough to have a sufficient area for the adhesive. The use of a resin to fix a quartz plate to a package fails to provide a piezoelectric acoustic wave device with long-term stability.